Several biological applications involve the selective amplification of nucleic acid molecules within a population. For example, next-generation sequencing methods can involve the analysis of selected targets within a large population of nucleic acid molecules. For such applications, it can be useful to increase the total number of targets that can be selectively amplified from a population within a single amplification reaction. Such selective amplification is typically achieved through use of one or more primers that can selectively hybridize to, or selectively promote the amplification of, a particular target nucleic acid molecule. Such selective amplification can be complicated by the formation of amplification artifacts, such as primer-dimers and the like. The formation of such amplification artifacts (also referred to herein as nonspecific amplification products) can consume critical amplification reagents, e.g., nucleotides, polymerase, primers, etc. Furthermore, such artifacts can frequently have shorter length relative to the intended product and in such situations can amplify more efficiently than the intended products and dominate the reaction output. Selective amplification can also be complicated by the formation of ‘superamplicons’, i.e., the formation of a extended amplicon, which can occur when extension of a first primer is extended through an adjacent target nucleic acid sequence, thereby creating a long non-specific amplification product, which can act as a template for extension with a second primer. The formation of such artifacts in amplification reactions, even when only a single pair of primers is employed, can complicate downstream applications such as qPCR, cloning, gene expression analysis and sample preparation for next-generation sequencing. In some downstream applications, including several next-generation sequencing methods, this problem can be compounded by the requirement to practice a secondary amplification step, since the artifacts can be further amplified during the secondary amplification. For example, downstream sequencing applications can involve the generation of clonally amplified nucleic acid populations that are individually attached to separate supports, such as beads, using emulsion PCR (“emPCR”) and enrichment for clonal amplicons performed via positive selection. In such applications, the artifacts can be carried all the way through the library generation process to the emPCR stage, producing DNA capture beads that include non-specific amplification products. These artifact-containing beads can be selected for during the enrichment process with the template containing beads but are genetically non-informative.
Nucleic acid molecules amplified in a multiplex PCR reaction can be used in many downstream analysis or assays with, or without, further purification or manipulation. For example, the products of a multiplex PCR reaction (amplicons) when obtained in sufficient yield can be used for single nucleotide polymorphism (SNP) analysis, genotyping, copy number variation analysis, epigenetic analysis, gene expression analysis, hybridization arrays, analysis of gene mutations including but not limited to detection, prognosis and/or diagnosis of disease states, detection and analysis of rare or low frequency allele mutations, nucleic acid sequencing including but not limited to de novo sequencing or targeted resequencing, and the like.
Exemplary next-generation sequencing systems include the Ion Torrent PGM™ sequencer (Life Technologies) and the Ion Torrent Proton™ Sequencer (Life Technologies), which are ion-based sequencing systems that sequence nucleic acid templates by detecting ions produced as a byproduct of nucleotide incorporation. Typically, hydrogen ions are released as byproducts of nucleotide incorporations occurring during template-dependent nucleic acid synthesis by a polymerase. The Ion Torrent PGM™ sequencer and Ion Proton™ Sequencer detect the nucleotide incorporations by detecting the hydrogen ion byproducts of the nucleotide incorporations. The Ion Torrent PGM™ sequencer and Ion Torrent Proton™ sequencer include a plurality of nucleic acid templates to be sequenced, each template disposed within a respective sequencing reaction well in an array. The wells of the array are each coupled to at least one ion sensor that can detect the release of H+ ions or changes in solution pH produced as a byproduct of nucleotide incorporation. The ion sensor comprises a field effect transistor (FET) coupled to an ion-sensitive detection layer that can sense the presence of H+ ions or changes in solution pH. The ion sensor provides output signals indicative of nucleotide incorporation which can be represented as voltage changes whose magnitude correlates with the H+ ion concentration in a respective well or reaction chamber. Different nucleotide types are flowed serially into the reaction chamber, and are incorporated by the polymerase into an extending primer (or polymerization site) in an order determined by the sequence of the template. Each nucleotide incorporation is accompanied by the release of H+ ions in the reaction well, along with a concomitant change in the localized pH. The release of H+ ions is registered by the FET of the sensor, which produces signals indicating the occurrence of the nucleotide incorporation. Nucleotides that are not incorporated during a particular nucleotide flow will not produce signals. The amplitude of the signals from the FET may also be correlated with the number of nucleotides of a particular type incorporated into the extending nucleic acid molecule thereby permitting homopolymer regions to be resolved. Thus, during a run of the sequencer multiple nucleotide flows into the reaction chamber along with incorporation monitoring across a multiplicity of wells or reaction chambers permit the instrument to resolve the sequence of many nucleic acid templates simultaneously. Further details regarding the compositions, design and operation of the Ion Torrent PGM™ sequencer can be found, for example, in U.S. patent application Ser. No. 12/002,781, now published as U.S. Patent Publication No. 2009/0026082; U.S. patent application Ser. No. 12/474,897, now published as U.S. Patent Publication No. 2010/0137143; and U.S. patent application Ser. No. 12/492,844, now published as U.S. Patent Publication No. 2010/0282617, all of which applications are incorporated by reference herein in their entireties. In some embodiments, amplicons can be manipulated or amplified through bridge amplification or emPCR to generate a plurality of clonal templates that are suitable for a variety of downstream processes including nucleic acid sequencing. In one embodiment, nucleic acid templates to be sequenced using the Ion Torrent PGM™ or Ion Torrent Proton™ system can be prepared from a population of nucleic acid molecules using one or more of the target-specific amplification techniques outlined herein. Optionally, following target-specific amplification a secondary and/or tertiary amplification process including, but not limited to a library amplification step and/or a clonal amplification step such as emPCR can be performed.
As the number of nucleic acid targets desired to be amplified within a sample nucleic acid population increases, the challenge of selectively amplifying these targets while avoiding the formation of undesirable amplification artifacts can correspondingly increase. For example, the formation of artifacts including primer-dimers and superamplicons can be a greater issue in multiplex PCR reactions where PCR primer pairs for multiple targets are combined in a single reaction tube and co-amplified. In multiplex PCR, the presence of additional primer pairs at elevated concentrations relative to the template DNA makes primer-primer interactions, and the formation of primer-dimers and other artifacts, more likely.
Current methods for avoiding or reducing the formation of artifacts, such as primer-dimers, during nucleic acid amplification center around the primer design process and often utilize dedicated software packages (e.g., DNAsoftwares's Visual OMP, MultiPLX, ABI's Primer Express, etc.) to design primer pairs that are predicted to exhibit minimal interaction between the other primers in the pool during amplification. Through the use of such software, primers can be designed to be as target-specific or amplicon-specific as possible, and often are grouped into subsets to minimize primer-primer interactions, primer-dimer formation and superamplicons. Stringent design parameters, however, limit the number of amplicons that can be co-amplified simultaneously and in some cases may prevent the amplification of some amplicons altogether. Other current methods require the use of multiple PCR primer pools to segregate primers into non-overlapping pools to minimize or prevent primer artifacts during the amplification step. Other methods include the use of multiple primer pools or single plex reactions to enhance the overall yield of amplification product per reaction. In a multiplex PCR reaction, each primer pair competes in the amplification reaction with additional primer pairs for a finite amount of dNTPs, polymerase and other reagents. There is therefore a need for improved methods, compositions, systems, apparatuses and kits that allow for the selective amplification of multiple target nucleic acid molecules within a population of nucleic acid molecules while avoiding, or minimizing, the formation of artifacts (also referred to as non-specific amplification products), including primer dimers. There is also a need for improved methods, compositions, systems, apparatuses and kits that allow for the selective amplification of multiple target nucleic acid molecules from a single nucleic acid sample, such as genomic DNA and/or formalin-fixed paraffin embedded (FFPE) DNA while avoiding, or minimizing, the formation of artifacts. There is also a need in the art for improved methods, compositions, systems and kits that allow for the simultaneous amplification of thousands of target-specific nucleic acid molecules in a single reaction, which can be used in any applicable downstream assay or analysis.
The practice of the present subject matter may employ, unless otherwise indicated, conventional techniques and descriptions of organic chemistry, molecular biology (including recombinant techniques), cell biology, and biochemistry, which are within the skill of the art. Such conventional techniques include, but are not limited to, preparation of synthetic polynucleotides, polymerization techniques, chemical and physical analysis of polymer particles, preparation of nucleic acid libraries, nucleic acid sequencing and analysis, and the like. Specific illustrations of suitable techniques can be used by reference to the examples provided herein. Other equivalent conventional procedures can also be used. Such conventional techniques and descriptions can be found in standard laboratory manuals such as Genome Analysis: A Laboratory Manual Series (Vols. I-IV), PCR Primer: A Laboratory Manual, and Molecular Cloning: A Laboratory Manual (all from Cold Spring Harbor Laboratory Press), Hermanson, Bioconjugate Techniques, Second Edition (Academic Press, 2008); Merkus, Particle Size Measurements (Springer, 2009); Rubinstein and Colby, Polymer Physics (Oxford University Press, 2003); and the like.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which these inventions belong. All patents, patent applications, published applications, treatises and other publications referred to herein, both supra and infra, are incorporated by reference in their entirety. If a definition and/or description is set forth herein that is contrary to or otherwise inconsistent with any definition set forth in the patents, patent applications, published applications, and other publications that are herein incorporated by reference, the definition and/or description set forth herein prevails over the definition that is incorporated by reference.
As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).